Clock data recovery (CDR) circuits are often used in high speed signaling applications to recover clock and data signals from a signal line. Typically, transitions in an incoming signal are detected and used to generate a clock signal which is used, in turn, to sample data in the incoming signal.
FIG. 1 illustrates a prior art integrated circuit device 101 that includes a number of CDR circuits, CDR0–CDRN-1, to recover clock and data signals from lines DATA0–DATAN-1, respectively. Each CDR circuit includes a receiver 102 and a clock generator 103. The receiver 102 captures a number of samples of the incoming signal during each cycle of the recovered clock signal 104. The samples are compared with one another within a phase control circuit 107 (shown in the detail of clock generator 103 of CDR0) to determine whether transitions in the incoming signal occur early or late relative to edges of a recovered clock signal 104. Based on the early/late determination, the phase control circuit 107 outputs a control signal to a phase adjust circuit 105 which responds by advancing or delaying (i.e., retarding) the phase of the recovered clock signal 104. Eventually, each of the CDR circuits reaches a phase locked condition in which the recovered clock signal 104 has a desired phase relationship to the incoming data signal (e.g., the recovered clock signal becomes aligned with the midpoint of the data eye), and the phase control output begins to toggle between advancing and delaying the phase of the recovered clock signal 104.
One disadvantage of the CDR arrangement in the integrated circuit 101 is that each of the signals on lines DATA0–DATAN-1 usually must undergo some minimum number of transitions per unit time (i.e., have a minimum transition density) in order for the corresponding CDR circuit to remain phase locked. Because the data content of the incoming signal may include relatively long sequences of zeroes or ones which lack such transitions, encoding, such as 8b/10b encoding, is often performed to replace the incoming data with codewords that meet the minimum transition density requirement. Because the codewords typically include more bits than the data values they replace, signal encoding tends to consume bandwidth on the signal line that could otherwise be used to transmit data. For example, in the case of 8b/10b encoding, each eight-bit data value is replaced with a 10-bit codeword, so that the encoding scheme consumes 20% of the signal line bandwidth. Other data encoding schemes also tend to consume signal line bandwidth.